I play a lot of mewtwo (I went undefeated for awhile at Fight or Flight) his fair while hard to hit, is very good and his nair is great for racking up damage over time. his bair can edge guard somethings and his dair while not great can still do something sometimes haha. but i think the main reason i put him higher is because DK doesnt have many approach options and not very many throws besides back throw and up throw. His arieals are lacking even his good ones like nair and bair and possibly uair dont do much or combo into anything. Not to mention he is huge and has literally the worst recovery (Vertical at least) in the game. He is combo bait while mewtwo yes being light can still be hard to combo due to that. I think i mainly put him just below mewtwo because I personally have seen WAY more success with mewtwo than DK when i play against Fox, Falco, Marth and Shiek
I'll go through this point by point:
First with the Mewtwo discussion:
1. His Bair hitbox is super wonky, hard to hit, and most of it has so little KB that it can actually help your opponent's recovery.
2. His Nair can be SDI'd and does very little damage.
3. His Dair can be useful-ish if you learn how to use it, but it's about as good as DK's
Then about DK:
1. DK has about as many approach options as Mewtwo
2. "Not many throws", meaning half of his throws. B-Throw has pretty good knockback, Cargo D-Throw is a good mixup if they are expecting Cargo U-Throw and thus DI incorrectly. If they do that, it has kill potential. U-Throw is mediocre, but Cargo U-Throw is one of the best juggle setups in the game.
3. Nair has really good KB, Bair really disjointed, Dair is decent if you think Mewtwo's is good, Fair is pretty horrid, and Uair is one of (if not the) best juggling move in the game. DK's main problem is his lag is pretty bad, not any of his aerials.
4. His Up-B is pretty bad. I agree.
5. While Mewtwo is light, he gets killed WAY earlier than DK, and both of them can still get comboed fairly easily by most characters (if your opponent is decent at comboing).
6. What you have personally seen at your level doesn't really matter, what I've personally seen at my level doesn't matter. What we see at the top level is what we should be considering and while I have Green Ranger, Phish-It, Strongbad, and Chandy playing to back up DK being higher than Mewtwo on the tier list based upon performance against top tiers of a decent level, who do you have backing up your evidence?